A conventional rotary head-drum assembly having a rotating top drum and adapted for use in a video tape recorder making high-density recording is described in Japanese Utility Model Laid-Open No. 89449/1984.
As shown in FIGS. 3 and 4, this conventional rotary head-drum assembly comprises a rotating top drum 2 and a stationary bottom drum 4. Both drums 2 and 4 are mounted on the same axis. A plurality of magnetic transducing heads 1 are mounted near the lower end of the outer surface of the rotary drum 2. A helical guide band 3 is formed on the outer periphery of the fixed drum 4. The diameter D.sub.1 of the rotary drum 2 is set larger than the diameter D.sub.2 of the fixed drum 4.
Magnetic tape 5 is wound helically on the outer surfaces of the drums 2 and 4. The rotary drum 2 is rotated, for example, in the direction indicated by the arrow b while moving the tape 5 helically, for example, in the direction indicated by the arrow a along the guide band 3. Thus, as shown in FIG. 5, the plural heads 1 record or play back slanted recording tracks TR on the tape 5.
In this kind of rotary head-drum assembly, the magnetic tape 5 is in contact with the assembly for about 210.degree.. Head switch occurs at every 180.degree.. Therefore, the heads 1 comprise some pairs of heads. In each pair, two heads are mounted in a diametrically opposite relation to each other. The heads 1 comprise a pair of recording heads, a pair of video playback heads, a pair of audio playback heads, and a pair of erase heads.
As is well known in the art, in the rotary head-drum assembly of this kind, an air layer 8 which gradually decreases in thickness from the side of the entrance 6 of the drum assembly to the side of the exit 7 of the drum assembly is formed between the rotary drum 2 and the tape 5, as indicated by the hatching in FIG. 4. In the rotary head-drum assembly of a video tape recorder making high-density recording, the rotary drum 2 rotates at a very high frequency of 90 Hz and so the air layer 8 is very thick. Especially, on the side of the entrance 6 of the drum assembly, the layer 8 is as thick as 30 to 40 .mu.m.
Furthermore, in the above-described prior art rotary head-drum assembly, the diameter D.sub.1 of the rotating upper drum 2 is set larger than the diameter D.sub.2 of the stationary lower drum 4. Therefore, as shown in FIG. 5, the length L.sub.1 of the portion of the upper edge 5a of the tape 5 which is wound around the rotary drum 2 via the air layer 8 is larger than the length L.sub.2 of the portion of the lower edge 5b of the tape that is wound on the fixed drum 4 between the entrance 6 and the exit 7 of the drum assembly. This makes it impossible to make uniform the distribution of the elongation coefficient in the direction of the width indicated by W. The coefficient is the percentage of the elongation of the length L.sub.1 of the wound portion of the upper edge 5a of the tape relative to the length L.sub.2 of the wound portion of the lower edge 5b of the tape, i.e., ##EQU1##
Therefore, in the prior art rotary head-drum assembly, the air layer 8 at the side of the entrance 6 of the drum assembly is thick, and the length L.sub.1 of the wound portion of the upper edge 5a of the tape 5 is increased. Consequently, the heads 1 are pressed into the tape 5 with reduced force at the side of the entrance 6 of the drum assembly. This reduces the head output power.